This research seeks to identify the consequences of exposure to PCBs and other toxins in the environment on the health of children and youth as measured by their physical growth and maturation and by their cognitive/behavioral development. There are two general objectives of this research, 1) to identify the effects of PCB body burden on endocrines that reflect thyroid function and sexual maturation, and on the physical growth, maturation and cognitive development of children and youth and, 2) to identify the role of body composition, specifically fat stores, in determining the level of PCBs in circulation and as a modifier of their effects. A cross-sectional study of mohawk children and youth 10-16 years of age (anticipated n=400) will permit determination of the relationships of PCB level to endocrines (thyroxine, triiodothyroine, thyrotropin, and beta-estradiol / testosterone), physical growth, maturation and cognitive- behavioral functioning as well as the influence of fat mass or percent fat on PCB levels and PCB effects. Exposure to PCBs is determined by serum assay, and the primary outcomes are assessed by direct examination (anthropometry and several measures of cognitive and behavioral functioning), or in the case of endocrines, by blood analysis. Body composition (fat mass, percent fat) is determined using multiple anthropometric measures (primarily skinfolds, circumferences and bony breadths). The study will also collect information on other factors that affect the outcomes of interest and, when necessary, control them through statistical methods. The study will determine levels of blood mercury (organic and inorganic), lead, and the level of fluoride in urine because the population at Akwesasne are exposed to these toxins that may affect growth and development. Other influences (for example, diet, socio- economic status, birth order, and others) are assessed through questionnaire. Iron status is assessed through blood assay. Multiple regression techniques, particularly structural equation analysis, of these data will permit control of the known influences on the outcomes of interest and the estimation of direct and indirect effects of PCBs on the intermediate and final outcomes of interest.